Infusion pouches, for example teabags and spice-bags are commonly formed as pouches of a non-woven material (referred to hereinafter as "teabag paper") that is permeable to water and to the beverage formed by infusion, i.e. by the dissolution of soluble solids in the contents of the pouch, upon the application of hot water thereto.
Teabag paper is generally a non-woven web of a light weight permeable fibrous material made, for example, from abaca pulp, sisal pulp, regenerated rayon, esparto grass pulp, long-fibred chemical wood pulp or mixtures thereof. In order to permit the fabrication of a heat-sealed pouch, the fibrous material may comprise heat-sealable fibres such as polyolefins, e.g. polyethylene or polypropylene, or vinyl chloride and vinyl acetate polymers or copolymers. The heat-sealable fibres may constitute a discrete phase on, for example, a cellulosic base phase.
Teabag paper is currently available in two types. One is a plain, non-woven web which is made on an ordinary Fourdrinier wire. The other type is a patterned web, the pattern being formed by an array of discrete areas having a lower fibre density than that of the rest of the web.
Teabag paper of the second type is formed on a wire having pronounced knuckles, as described in British Patent Specification No. 1,102,246. However, in the course of manufacturing the web, the knuckles of the wire often break through the web and give rise to clear holes of the size of the knuckle.
It is also known that perforated or reticulated non-woven materials can be produced by forming a wet-laid web, supporting this on a perforated screen and forcing jets of fluid through the supported web. Such techniques are disclosed in Bristish Patent Specifications No. 836,397 and No. 1,326,915, and U.S. Pat. No. 3,485,706.
To be completely acceptable, teabag paper must possess characteristics such as cleanliness, good absorbency, high wet strength and a sheet structure that permits rapid permeation of the beveravage; it is also found that many consumers have a preference for teabags formed from paper having a pattern thereon. However, it is also important that the paper should not sift, that is it should prevent the passage therethrough of fine particles ("dust") of the tea or other solids contained in the bag or pouch. Clearly, however, the presence of clear holes in the web will cause sifting of the web. If one surveys the filtering media produced by prior-art methods, it is found that they fall within the following categories: (i) products with a good pattern difinition but poor dust-retention properties, (ii) products with good dust-retention properties but a poorly defined pattern and (iii) products with mediocre pattern definition and mediocre dust-retention properties.
Accordingly, there is a definite need for a patterned or decorative filter medium having a good pattern definition coupled with good filtration or sifting characteristics.
In the following text, the invention will be discussed primarily in terms of teabag paper; however, it should be understood that the invention can be applied to other non-woven filtration media, for example non-woven fabrics used in surgical face masks, coffee filters and the like.